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Q & A: Tin can telephones

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Most recent answer: 10/22/2007
Q:
I am doing a scientific investigation with elementary students. We mad several tin-can telephones with different materials for the string. We used fishing line, plastic twine, plastic lanyard, and cotton twine. The cotton twine worked the best...do you know why?
- Andrea (age 24)
Project Teach, Seattle, Washington
A:
Hi Andrea,

I cannot say for sure what may be affecting your setup, but I can give some suggestions for factors that may be causing some of the differences you can hear.

The sound strength and quality can be degraded by the connecting string in a variety of ways, and these will in general depend on the tension in the string and the length of the string.

One is that the string can be "lossy". If the string stretches, it should be able to recover its original length and position very quickly without "oozing" back into place. The times which are important are the frequency of the human voice, around 300 Hz. Some of the plastics might perform less well than other materials. If you flex a material and it heats up instead of springing back into place, you can lose energy in the transmission process.

The string should not be too "dispersive". Some materials will transmit high frequencies at a much faster rate than low frequencies. Take a metal slinky, for example, and hook one end over a piece of cardboard. Stretch it out and tap the other end with your fingernail. You will hear a chirping sound which starts off with high frequencies and ends up with low frequencies. Using a material that does this in the transmission of human voices will distort the sound, making it unintelligible.

The string should "couple well" to the can. If you tie a knot in a piece of fishing line to hold it in place in a hole in the can, the knot itself can flex and dissipate energy instead of transmitting it to the wall of the can.

Tom

(published on 10/22/2007)

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